1. Field of Invention
The present invention relates generally to catheters and, more particularly, to catheters having piston-type actuators.
2. Description of the Related Art
Catheters, which are in widespread medical use today, allow physicians to gain access into interior regions of the body in a minimally invasive manner. In cardiac treatment, for example, the catheter is steered through a main vein or artery into the region of the heart that is to be treated. Once the catheter is within the desired region, additional steering is required to place the operative element (such as an electrode) carried on the distal portion of the catheter into direct contact with the desired tissue.
Although precise control of catheter movement is of paramount importance in all catheter-based procedures, the need for careful and precise control over the catheter is especially critical during certain procedures concerning tissue within the heart. These procedures, called electrophysiological therapy, are becoming more widespread for treating cardiac rhythm disturbances. Cardiac ablation is one procedure in which ability to precisely bend and shape the distal end of the catheter is especially important. Incremental distal end movements of 1 mm to precisely position ablation electrode(s) are not uncommon and it can take up to an hour to precisely position the catheter tip.
Conventional catheters typically include an elongate, hollow catheter body and a handle secured to the proximal end of the catheter body. Piston-type handles include a thumb-actuated inner piston which can be moved relative to the handle. The piston is connected to the hollow catheter body such that distal movement of the piston forces the catheter body in the distal direction. A steering wire passes through the hollow catheter body. The proximal end of the steering wire is fixedly anchored to the handle, while the distal end of the steering wire is secured to the distal end (or tip) of the catheter body. In many instances, the steering wire is connected to a flat leaf spring located within the distal tip of the catheter body. The flat leaf spring makes the distal tip more flexible than the remaining portion of the catheter body.
When the piston is moved distally, the steering wire pulls on the distal end of the catheter body, thereby causing the distal end of the catheter body to deflect into a curved orientation. Moving the piston proximally allows the catheter body to return to its original orientation. Accordingly, physicians are able to steer the catheter by moving the piston back and forth as the catheter is advanced into the patient. Manipulation of the piston also enables the distal tip to be positioned as desired within the patient. The range of piston motion (or "stroke") corresponds to the distance that the steering wire can be displaced prior to causing the distal end of the catheter to "over curve" or "pigtail." A range of motion of approximately 0.4 inch is typical.
When the distal end of the catheter body is bent, a force is generated that tends to drive the distal end back to its straightened orientation. This force, which is generated by the flat leaf spring, also tends to drive the piston in the proximal direction. In order to hold the piston in various locations along its stroke and prevent the distal end of the catheter body from straightening out when the physician releases the piston, the handle includes a set screw that applies a friction force to the piston. The friction force applied to the piston by the set screw must, of course, be greater than the maximum force generated by the distal end of the catheter body. The maximum force is generated when the piston is in its distal-most position and the distal end of the catheter body is in its most curved orientation. The maximum force generated by the distal end of the catheter body is approximately 4 lbs. in many conventional catheters and the amount of friction force required to hold the piston is, therefore, slightly more than 4 lbs. The distal end force will, of course, be greater is some types of catheters than it will in others, as will the friction force.
Despite the aforementioned need for precise catheter control, the inventors herein have determined that conventional piston-type handles suffer from a number of shortcomings which make it difficult for the physician to precisely control the curvature of the catheter. For example, a relatively large static friction force created by the set screw, as well as the force generated by the leaf spring, must be overcome just to get the piston moving. This phenomenon is illustrated in FIG. 1. As noted above, the physician drives the piston with his or her thumb. Generating an actuation force sufficient to overcome the friction force created by the set screw is not only uncomfortable, it also tends to make movement of the piston unsteady (or jerky). This is especially true when the physician begins to drive the piston from an at rest state to the location corresponding to the next desired distal end curvature. In instances where incremental movements of 1 mm may be required, unsteady movement of the piston often leads to missed targets and adds unnecessarily to the amount of time which it takes to position the catheter.
The inventors herein have also determined that another problem associated with conventional piston-type handles is that the displacement of the catheter tip is not linear over the piston stroke. Moving the piston 0.1 inch when the piston is close to its proximal-most position will lead to less distal tip movement than 0.1 inch of piston movement when the piston is close to its distal-most position. In other words, movement of the distal tip accelerates with distally directed movement of the piston and decelerates with proximally directed movement. This phenomenon is illustrated in FIG. 11 in that the relationship between piston displacement and distal tip displacement is non-linear and there is a great disparity between the respective piston displacement to tip displacement relationships at the distal and proximal ends. The inconsistent relationship between piston displacement and distal tip displacement makes it difficult for the physician to accurately position the distal end of the catheter.
Moreover, the actuation force is greatest when the piston is being pushed distally near the distal-most end of the stoke, where the physician's thumb is in the most extended position. This adds to the difficulty associated with the accurate positioning of the distal end of conventional catheters.
The inventors herein have further determined that it would be desirable to increase the stroke of the piston in order to improve control of the catheter tip. Unfortunately, as noted above, the piston stroke in conventional catheters is limited by the distance that the steering wire can be displaced prior to causing the distal end of the catheter to "over curve" or "pigtail."